Inspector Baynes of the Surrey force appears in the two-part series "The Adventure of Wisteria Lodge", named (i) "The singular experience of Mr John Scott Eccles", and (ii) "The Tiger of San Pedro". He is the only uniformed policeman in the books to have ever matched Sherlock Holmes in his investigative skills. In this story, the reader finds that even despite working in different lines, they both arrive at the right conclusion and solve the mystery at the same time. In fact, Baynes had misled even Holmes as he used a method similar to one that Holmes often used when he arrested the wrong man and provided inaccurate information to the press in order to lull the true criminal into a false sense of security. Holmes congratulated this inspector and believed that he would go far.

In Japanese puppetry Sherlock Holmes, Baynes is a pupil of Beeton School as well as Holmes and has a strong sense of rivalry against him. Baynes speaks in a precocious manner and provokes Holmes to find the truth of the disappearance of two pupils, Garcia and Henderson. After that, he provokes Holmes again by posting a message using the stick figures of dancing men in the school. Yōsuke Asari voices him.

Doyle described him as "a tall, stout official... in a peaked cap and frogged jacket". Sidney Paget's illustrations for the Strand Magazine depict him with a full beard. Beyond this little is revealed about him in the canon.

Bradstreet originally served in Scotland Yard's E Division which associates him with the Bow Street Runners, a forerunner of Scotland Yard. He claims to have been in the force since 1862 ("The Man with the Twisted Lip") but in June 1889 Dr Watson writes he is in B Division to oversee "The Adventure of the Blue Carbuncle". According to Sherlockian author Jack Tracy, B Division was:

In "The Adventure of the Engineer's Thumb", he accompanied Holmes to Eyford, a village in Berkshire. According to Jack Tracy's The Encyclopaedia Sherlockiana, he was "assigned most likely to the central headquarters staff."

Bradstreet is not a martinet; in "The Man with the Twisted Lip" he could have prosecuted the false beggar, but chose to overlook this action to spare Neville St Clair the trauma of shaming his wife and children.

Gregson first appears in A Study in Scarlet and is a polar opposite of another Yarder Doyle created, Inspector Lestrade. Lestrade and Gregson are such visual opposites, it indicates the barrier Doyle drew between them to emphasise their professional animosity. Gregson is tall, "tow-headed" (fair-haired) in contrast to the shorter Lestrade's dark "ferretlike" (narrow) features and has "fat, square hands".

Of all the Yarders, Gregson comes the closest to meeting Sherlock Holmes on intellectual grounds, while acknowledging Holmes's abilities. He even admits to Holmes that he always feels more confident when he has Holmes's aid in a case. Regrettably, he is bound within the confines of the law he serves, and the delay in getting his assistance turns to tragedy in "The Adventure of the Greek Interpreter". He also has some regrettable human flaws. During A Study in Scarlet he publicly laughs at Lestrade's incorrect assumptions, even though he is also on the wrong trail.

Unlike Lestrade, Gregson overlooks the little grey areas of the law, and in "The Adventure of the Greek Interpreter" overlooks Holmes's breaking of a window in order to enter a premises. The life of Mycroft Holmes's fellow lodger is saved by this minor criminal act.

Gregson last appears in Doyle's "The Adventure of the Red Circle" in events that happen in 1902 but are not published by Dr Watson until 1911. In this story, Watson observes that:

Our official detectives may blunder in the matter of intelligence, but never in that of courage. Gregson climbed the stair to arrest this desperate murderer with the same absolutely quiet and businesslike bearing with which he would have ascended the official staircase of Scotland Yard. The Pinkerton man had tried to push past him, but Gregson had firmly elbowed him back. London dangers were the privilege of the London force.

A character named Captain Gregson of the NYPD appears in the TV adaptation Elementary, portrayed by Aidan Quinn. Originally he was to be called Tobias Gregson, after the character in the stories, but his name was changed to Thomas Gregson.[4]

Inspector Stanley Hopkins is a Scotland Yard detective and a student of Holmes's deductive methods, who attempts to apply them in his own investigations. Holmes, however, is very critical of Hopkins's ability to apply them well, Hopkins sometimes making such mistakes as arresting a man whose notebook was found at a crime scene despite it being physically impossible for the man in question to have killed the victim in the manner that he was discovered. Hopkins refers several cases to Holmes, all within the South-East areas of England and London, including:

Mrs Hudson is a woman who wants the home to be clean and tidy, and often fights with Holmes for this. Watson describes her as a very good cook; in "The Naval Treaty," Holmes says "Her cuisine is a little limited, but she has as good an idea of breakfast as a Scotchwoman,"[5] which some readers have taken to mean that she is Scottish, and others that she cannot possibly be. Other than one mention of her "queenly tread", she is given no physical description or first name, although she has been identified with the "Martha" in "His Last Bow".[6][7]

Watson described the relationship between Holmes and Hudson in the opening of "The Adventure of the Dying Detective":

Mrs Hudson, the landlady of Sherlock Holmes, was a long-suffering woman. Not only was her first-floor flat invaded at all hours by throngs of singular and often undesirable characters but her remarkable lodger showed an eccentricity and irregularity in his life which must have sorely tried her patience. His incredible untidiness, his addiction to music at strange hours, his occasional revolver practice within doors, his weird and often malodorous scientific experiments, and the atmosphere of violence and danger which hung around him made him the very worst tenant in London. On the other hand, his payments were princely. I have no doubt that the house might have been purchased at the price which Holmes paid for his rooms during the years that I was with him.

The landlady stood in the deepest awe of him and never dared to interfere with him, however outrageous his proceedings might seem. She was fond of him, too, for he had a remarkable gentleness and courtesy in his dealings with women.[8]

At one point in "A Scandal in Bohemia" Holmes calls the landlady "Mrs Turner", rather than Mrs Hudson, which has caused much speculation among Holmes fans.[9]

In film and television adaptations of the stories,[10] Mrs Hudson is usually portrayed as an older woman; on rare occasions she is presented as a young woman.

In the BBC series Sherlock, she is played by actress and TV presenter Una Stubbs. She offers Holmes a lower rent because he helped her out by ensuring the conviction and execution of her husband in Florida after he murdered two people. In "A Scandal in Belgravia" when agents torture Mrs Hudson trying to find a mobile phone, Sherlock repeatedly throws the agent responsible out of an upper-level window, and later states that "England would fall" if Mrs Hudson left Baker Street. In "His Last Vow" her name is revealed to be Martha Louise Hudson (née Sissons), a semi-reformed alcoholic and former exotic dancer. Her "pressure point", according to Charles Augustus Magnusson's information on her, is marijuana.

A transgender Miss Hudson appears in the 19th episode of the US series Elementary as an expert in Ancient Greek who essentially makes a living as a kept woman and muse for various wealthy men; Holmes allows her to stay in the apartment after a breakup, and she subsequently agrees to clean for them once a week as a source of income and to prevent Holmes having to do it himself.

Shinwell "Porky" Johnson is a former criminal who acts as informant and occasional muscle for Sherlock Holmes (Although Watson notes that he is only useful in cases that by their nature will not go to court as he would compromise his connection to Holmes and thus render himself useless as a source if he ever had to testify as part of a case). He appears in "The Adventure of the Illustrious Client" where he protects Kitty from Baron Grüner's henchmen and provides Holmes with insight into how he might go about infiltrating Grüner's house to acquire a certain book. He is referred to in the radio adaptations of Sherlock Holmes, specifically in The Further Adventures of Sherlock Holmes: The Ferrers Documents where he appears to carry on with intimidation business.

The fifth season of the TV show Elementary introduced an updated version of the character (played by Nelsan Ellis) as both a former patient of Watson's and ex-convict now attempting to go straight.

Inspector Athelney Jones is a Scotland Yard detective who appears in The Sign of the Four. He arrests the entire household of Bartholomew Sholto, including his brother and servants, on suspicion of his murder, but is forced to release all but one of them, much to his own embarrassment.

An Inspector Peter Jones appears in "The Adventure of the Red-Headed League"; it is unknown if he is related to Athelney. He does however refer specifically to events in The Sign of the Four, and Watson says that he speaks in a "consequential way", suggesting a connection. Holmes refers to Jones as "an imbecile" but also acknowledges him as being "tenacious as a lobster."

Mary Morstan is the wife of Dr. Watson. She is first introduced in The Sign of the Four, where she and Watson tentatively become attracted to each other, but only when the case is resolved is he able to propose to her. She is described as blonde with pale skin. At the time she hires Holmes she had been making a living as a governess. Although at the end of the story the main treasure is lost, she has received six pearls from a chaplet of the Agra Treasure.

Her father, Captain Arthur Morstan, was a senior captain of an Indian regiment and later stationed near the Andaman Islands. He disappeared in 1878 under mysterious circumstances that would later be proven to be related to the mystery, The Sign of the Four. Her mother died soon after her birth and she had no other relatives in England, although she was educated there (in accordance with the received wisdom of the time about children in the colony of India) until the age of seventeen. Shortly afterwards her father disappeared and she found work as a governess. Watson and Mary marry in 1889.

Amanda Abbington in the third season of Sherlock. She is the real-life partner of the show's Watson, Martin Freeman. She is first introduced in "The Empty Hearse" when Watson is attempting to propose. However, Sherlock interrupts and surprises John to let him know that he is not dead. This backfires as Watson is angry with Sherlock. However, Mary likes Sherlock and agrees to talk to Watson. After getting married, however, it is revealed she was an assassin in the past with the initials A.G.R.A., Sherlock admitting that he had deduced some degree of her secret due to her past only extending back a few years, musing that Watson would have only married a woman who appealed to his subconscious 'need' for danger. She and Watson are reconciled, and they are expecting a daughter as of the last episode of the series. In a subsequent special, Mary is shown to be less guarded about her past, using her skills from her former profession to assist Sherlock and vex Mycroft.

Anna Ishibashi voices Mary Morstan, a pupil of Archer House in the NHK puppetry Sherlock Holmes. In the show, her elder brother Arthur is attacked by someone and she requests Holmes to find the truth behind it. Meanwhile, Watson falls in love with her at first sight and tries to show her his braveness. But there is a rival called Jonathan Small who sent her a picture postcard every week before her entrance into Beeton School and writes a song "You are My Treasure" for her. Small joins a chorus band formed by Arthur and the Sholto twins but is betrayed by Arthur who changes the title to "Agra Treasure" and makes it his own.

Lexi Wolfe in popular viral spoof No Place Like Holmes by Ross K Foad, and in spin-off The Mary Morstan Mysteries.

Langdale Pike is a celebrated gossipmonger whose columns are published in numerous magazines and newspapers (referred to as the "garbage papers" by Watson). He's introduced in The Adventure of the Three Gables in which he helps Holmes learn the name of the woman who led Douglas Maberley to his demise, although he does not actually appear in the story itself and is only referred to by Watson who describes Pike as "strange" and "languid" and states that all of Pike's waking hours are spent "in the bow window of a St. James's Street club". His character has however been expanded on or fleshed out elsewhere. In William S. Baring-Gould's biography of Sherlock Holmes it is claimed that Pike is a college acquaintance of Holmes who encourages a young Holmes to try his hand at acting. Here his real name is given as 'Lord Peter'. In Peter Ling's radio play for the BBC Radio series, Pike's real name is said to be Clarence Gable. Here he is also an old school-friend of Holmes's and is nervous of strangers and reluctant to leave his club for this reason.[13] In the Granada television adaptation starring Jeremy Brett as Holmes meanwhile, Pike (played by Peter Wyngarde) is also apparently an old university friend of Holmes's. Here he claims to be the benevolent counterpart of Charles Augustus Milverton (the eponymous blackmailer of The Adventure of Charles Augustus Milverton), who suppresses more information than he exposes. Though Watson is rather scathing about Pike, Holmes is more sympathetic towards him, suggesting that Pike is isolated, much like Holmes himself. In the American television series Elementary, Pike appears in the first episode of the second season as one of Holmes' sources in London; details are not seen as Pike moves quickly when delivering a package to Watson.[14]

In the NHK puppetry Sherlock Holmes, Pike is a pupil of Beeton School and assists Holmes in his investigation. He also works as informant and is fast at his job but tight with money. Besides he sells photographs of girls to male pupils. Tomokazu Seki voices him.

Toby is a dog who is used by Sherlock Holmes. He is first introduced in The Sign of the Four and is described by Watson as an "ugly long haired, lop-eared creature, half spaniel and half lurcher, brown and white in colour, with a very clumsy waddling gait." Though used by Holmes, the dog belongs to Mr Sherman who keeps a menagerie of creatures at No. 3 Pinchin Lane in Lambeth. Toby lives at No. 7 within his house. Holmes states he would "rather have Toby's help than that of the whole detective force in London" and requests the dog by name.

Toby also featured in the novel Sherlock Holmes vs Dracula; or, The Adventures of the Sanguinary Count by Loren D. Estleman, when Watson and Holmes called on Toby to track Count Dracula after finding him in a meat-packing district – Dracula's carriage having rolled through a distinctive piece of rubbish – allowing the two to track Dracula to Watson's house in time to learn that he has abducted Mary Watson.

In the Holmes-esque The Great Mouse Detective, Toby is a Basset Hound and a permanent resident of 221b Baker Street. He is frequently used by Basil, the eponymous protagonist, as a means of transport and to pick up trails.

In the NHK puppetry Sherlock Holmes, Toby is kept by Sherman in a shed in Beeton School and assists Holmes in his investigation. In the series, Sherman is a female pupil who loves animals and communicates with them, unlike Mr Sherman in "The Sign of the Four". Though being a pupil of Baker House, she doesn't live in the house but in the shed with animals.

A dog also appears in the Guy Ritchie's Sherlock Holmes films, but the similarity stops at it being a dog and living with Sherlock. Its name is Gladstone, it's clearly an english bulldog and technically belongs to Watson. Sherlock doesn't use it as a hound, but as a subject for his experiments, especially those involving narcotics of some kind. Hence the running gag in the films "He's killed the dog again!"

Auguste Lupa is the son of Sherlock Holmes and Irene Adler. He appears in two pastiche novels by author John Lescroart, Son of Holmes (1986) and Rasputin's Revenge (1987). Lupa, a secret agent during the First World War, is strongly implied to be the younger version of fictional detective Nero Wolfe in the mystery series by Rex Stout.

Enola Holmes is the younger sister and youngest sibling of Sherlock and Mycroft Holmes. She appears in the series The Enola Holmes Mysteries by Nancy Springer and it could be inferred that she appears in the story The Copper Beeches as Violet Hunter, however there is not enough evidence to support the fact. Enola is a very independent and rebellious girl who likes to wear trousers while riding her bike. She becomes a Perditorian, or finder of lost things, when her mother runs away with the gypsies and her brothers try to send her to boarding school. Using her natural cunning which seems to be inherited by every member of the Holmes family, she creates multiple disguises on her quest to be reunited with her mother and evade her brothers.

Wold Newton family theorist Win Scott Eckert devised an explanation in his Original Wold Newton Universe Crossover Chronology[15] to reconcile the existence of Raffles Holmes with canonical information about Sherlock Holmes and A.J. Raffles, which fellow Wold Newton speculator Brad Mengel incorporated into his essay "Watching the Detectives." According to the theory, Holmes married Marjorie in 1883, and she died giving birth to Raffles later that year. Since Raffles and Holmes are contemporaries, it has been suggested that Marjorie was actually Raffles' sister.

Eckert further proposed in his Crossover Chronology that (1) Raffles Holmes was the same character as the "lovely, lost son" of Sherlock Holmes referred to in Laurie R. King's Mary Russell novels,[16] and (2) Raffles Holmes was the father of Creighton Holmes, who is featured in the collection of short stories The Adventures of Creighton Holmes by Ned Hubble.[17]

Mengel's online essay was revised for publication in the Eckert-edited Myths for the Modern Age: Philip José Farmer's Wold Newton Universe (MonkeyBrain Books, 2005), a collection of Wold Newton essays by Farmer and several other "post-Farmerian" contributors, authorised by Farmer as an extension of his Wold Newton mythos. He does not appear or is ever mentioned in any of the original stories of Sherlock Holmes and is not a creation of Doyle.

He was first proposed by William S. Baring-Gould who wrote in his biography "Sherlock Holmes of Baker Street" that Sherrinford was the eldest brother of Sherlock Holmes. Holmes once stated that his family were country squires, which means that the eldest brother would have to stay to manage the house. If Mycroft were the eldest then he couldn't play the role he does in four stories of the Sherlock Holmes canon, so Sherrinford frees them both. This position is strengthened by the fact that Mycroft's general position as a senior civil servant was a common choice among the younger sons of the gentry.

The character (as "Sherringford",) appears along with his brothers in the Virgin New AdventuresDoctor Who novel All-Consuming Fire by Andy Lane, where he is revealed to be the member of a cult worshipping an alien telepathic slug that is mutating him and his followers into an insect-like form; the novel culminates with Holmes being forced to shoot his brother to save Watson.

He also appears, accused of a murder that Sherlock must find him innocent of, in the Call of Cthulhuroleplaying game adventure The Yorkshire Horrors.[19] Sherrinford also appears in the Italian comic series Storie da Altrove (a spin-off of Martin Mystère) as the eldest brother, born nine years before him, of Sherlock himself.[20][21]

A third Holmes sibling, whose first name is not revealed, also appears as a reference by Mycroft in the BBC Sherlock series, in the third and last episode of the third season. It has been confirmed that Sherrinford will be mentioned or will be appearing in the final episode of the fourth series; teased by the name on a card at San Diego Comic Con.